The present invention relates to an amplification circuit provided with automatic gain control means. More particularly the invention relates to an amplification circuit in which there is improved sensitivity of automatic gain control and in which action of the automatic gain control means does not cause variation of the DC voltage level in the output stage of the amplification circuit.
In a conventional circuit for amplification of an AC signal input an AC signal source S supplies an input to the bases of a pair of NPN transistors Q2 and Q3 which constitute a differential amplifier A, and whose collectors connect respectively through resistors R2 and R3 to a line +Vcc which is positive with respect to ground, and to active elements in the next stage. In the particular example shown in FIG. 1 the bases of NPN transistors Q4 and Q5 are connected to source S, the collectors are connected to line +Vcc and the emitters are connected to ground through resistors R4 and R5 respectively. The circuit forms an emitter follower amplifier for buffer purposes, an amplified AC signal output being taken out across the emitters of transistors Q4 and Q5. The emitters of both transistors Q2 and Q3 connect to the collector of an NPN transistor Q1 whose emitter connects to ground through a resistor R1 and to whose base is supplied automatic gain control input, referred to below simply as an AGC input. The AGC input may be proportional to the output of the amplification circuit or to input volume, as required, and is supplied, suitably as a negative-going input, to transistor Q1 by known means not shown. In this circuit a lowered AGC input to the base of transistor Q1 results in reduction of collector current of transistors Q2 and Q3, i.e., in reduced gain by the amplification circuit.
However, this gain control action in a conventional circuit also results in considerable variation of the DC level in the output, and it is difficult to maintain a balance of the requisite values of characteristics of transistors Q2 and Q3, with the result that there is frequently distortion or clipping of signals in subsequent stages connected to the amplification circuit.